For many years, the problem of disposability has plagued industries that provide disposable items, such as diapers, wet wipes, incontinence garments and feminine care products. Ideally, when a disposable product is intended to be discarded in either sewer or septic systems, the product, or designated portions of the product, should “disperse” and thus sufficiently dissolve or disintegrate in water so as not to present problems under conditions typically found in either household or municipal sanitization systems. While headway has been made in addressing this problem, one of the weak links has been the inability to create economical nonwoven materials that are readily dispersible in water but still have sufficient in-use properties such as strength, thickness, opacity, absorbency, softness, flexibility, cleansing, ease-of-use, etc. to make consumer-acceptable products. These nonwoven materials may be formed by wet or dry (air) laying processes which generally provide a random plurality of fibers that are joined together adhesively and/or physically. Under appropriate conditions, some nonwoven materials can dissolve or disintegrate in water such as by simple dilution in an excess of water or dilution in excess water with the application of appropriate shear force.
U.S. patent application Ser. Nos. 10/830,558, 10/251,610, and 9/900,698 illustrate a number of approaches to forming adhesively-bonded dispersible nonwoven webs. U.S. Pat. No. 4,755,421 illustrates a variety of approaches to forming physically-bonded dispersible nonwoven fabrics, such as those formed by hydroentangling methods.
In general, ion-sensitive polymers are polymers that are insoluble in aqueous solutions containing at least about 0.3 wt % of inorganic salts but are soluble when diluted by water, such as tap water. Ion-sensitive polymers can be utilized as adhesive binders in making dispersible nonwoven and tissue-based flushable articles. Ion-sensitive polymers manufactured by solution polymerization methods known in the art, particularly in a reaction medium comprising organic solvents, are highly effective at delivering the required in-use strength and dispersibility requirements of the desired products. However, these solution polymerization processes are time-intensive and costly. Additionally, higher levels of the ion-sensitive polymers are often required to achieve the targeted in-use properties relative to the traditional crosslinking, non-dispersible binders used in current wet wipe technology. Considering these factors, the use of solution-polymerization-derived ion-sensitive polymers in flushable disposable products is cost-prohibitive despite their excellent performance properties.
In an attempt to counter the negative effects associated with solution-polymerization-derived ion-sensitive polymers, it has been found that solution blending lower-cost emulsion polymers, often referred to cobinders, with the ion-sensitive polymers can be accomplished which can allow reduced ion-sensitive polymer usage in the nonwoven material and hence reduce its cost. However, the amount of emulsion polymer cobinder that can be blended with the ion-sensitive polymers is often limited to less than 45% and more typically in the 20-35% range without resulting in significantly negative impacts to both product in-use properties and dispersibility. These negative impacts are due to generally poor bonding or interaction between the dissimilar ion-sensitive polymer and the emulsion polymer, as well as the generally more hydrophobic characteristics of emulsion polymers. Thus, the beneficial cost impact of an emulsion polymer is limited by the low amounts which can be used in the nonwoven material.
Emulsion polymerization methods offer a more cost effective approach to producing ion-sensitive polymers than solution-polymerization methods, but a generation of sufficient wet strength from emulsion-polymerization-derived ion-sensitive polymers is difficult. Such materials have been previously described as in U.S. Pat. No. 6,562,892, but materials demonstrate weak interparticle bonds, thus reducing its viability as a substitute.
Thus, there is a need for an ion-sensitive polymer which incorporates the performance advantages of a solution-polymerization-derived polymer with the cost advantages of an emulsion polymer. There is still a further need for a wet wipe that is dispersible but is cost effective.